Because the mount point is already set by hotplug. Try this:sudo mount /mnt/sda1

Alternatively, you can look in /mnt using emelfm (as super user) or ls it in a terminal and make sure hotplug has set the mount point for your flash drive. It should be sda1, sdb1, sdc1, etc., depending how many memory devices you use (with other numbers if any is partitioned). You can then right-click on the mount point and select mount.

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